An optical modulator modulates the intensity of incident light and outputs it. As a conventional example, there has been an optical modulator described in U.S. Pat. No. 5,311,360 and an article "Deformable Grating Optical Modulator" (Optics Letters, Vol. 17, No. 9, May 1, 1992) by O. Solgaard et al. This optical modulator modulates the intensity of light by utilizing the diffraction effect of light and has the advantage of being miniaturized and mass-produced in an IC process.
FIG. 32(a) is a plan view of an optical modulator described in the above-mentioned U.S. patent and article, and FIG. 32(b) is a cross-sectional view taken along a line K-K' in FIG. 32(a).
The optical modulator includes a silicon substrate 1001, a spacer layer 1002 made of a silicon oxide film formed in a peripheral region of the silicon substrate 1001, and a dielectric layer 1003. The dielectric layer 1003 is patterned to a plurality of minute dielectric beams 1004, and the dielectric beams 1004 float in a hollow space with both ends supported by the spacer layer 1002. The dielectric layer 1003 is made of a silicon nitride film rich in silicon, and its residual stress is reduced to about 200 MPa. The thickness of the spacer layer 1002 and the dielectric layer 1003 is set to be equal to 1/4 of a wavelength of light whose efficiency is to be controlled, i.e., light which is incident upon the optical modulator.
Openings 1005 each having a width equal to that of each dielectric beam 1004 are formed between the dielectric beams 1004. Furthermore, an Al reflective film 1006 which also functions as an electrode is provided above the substrate 1001. The reflective film 1006 is composed of upper reflective films 1007 formed on the surfaces of the dielectric beams 1004 and lower reflective films 1008 formed on the surface of the substrate 1001 through the openings 1005. The upper reflective films 1007 and the lower reflective films 1008 form a reflection-type grating.
The optical modulation principle of a conventional optical modulator having the above-mentioned structure will be described with reference to FIGS. 33(a) and (b). In these figures, components identical with those in FIG. 32 are denoted by the reference numerals identical with those in FIG. 32, and their description will be omitted.
FIG. 33(a) shows a state where a voltage is not applied between the reflective film 1006 and the substrate 1001. At this time, the difference in step between the upper reflective films 1007 and the lower reflective films 1008 is 1/2 of a wavelength of the incident light, and the difference in optical path between light reflected from the upper reflective films 1007 and light reflected from the lower reflective films 1008 is one wavelength. Therefore, the phases of these light beams are matched. Thus, the reflection-type grating functions as an ordinary mirror with respect to incident light 1010 which is incident upon the grating,, and the incident light 1010 becomes zero-th order diffracted light 1011 to be reflected to an incident side.
On the other hand, under the condition that a voltage is applied between the reflective film 1006 and the substrate 1001, the reflective film 1006 and the substrate 1001 forms a capacitor interposing the dielectric layer 1003 and an air layer 1012, and the reflective film 1006 is positively charged and the substrate 1001 is negatively charged. Since an electrostatic attracting force is affected between the charges, the dielectric beams 1004 are bent and attracted to the substrate 1001 until they come into contact with the substrate 1001, as shown in FIG. 33(b). At this time, the difference in step between the surfaces of the upper reflective films 1007 and those of the lower reflective films 1008 becomes 1/4 of a wavelength of the incident light, and the difference in optical path between the light reflected from the surfaces of the upper reflective films 1007 and the light reflected from the surfaces of the lower reflective films 1008 becomes a 1/2 wavelength in round travel, whereby the phases between these light beams are shifted by a half wavelength. Thus, the light reflected from the upper reflective film 1007 and the light reflected from the lower reflective film 1008 cancel each other to eliminate zero-th order diffracted light, and diffracted light other than the zero-th order diffracted light is output. For example, at this time, .+-.1st order diffracted light beams 1013a and 1013b are generated at a diffraction efficiency of 41%, respectively. As described above, the optical modulator is capable of modulating incident light by turning on/off a voltage applied to the reflective film 1006 and the substrate 1001.
However, the above-mentioned conventional optical modulator modulates incident light having a beam diameter at most with a size of the grating. Thus, in order to modulate incident light having a large diameter, it is required to increase the size of the grating. However, when the grating is increased in size, the grating is likely to adhere to the silicon substrate 1001 during the step of floating the grating by a half wavelength. Therefore, it was difficult to produce such a conventional optical modulator with a good yield.
The objective of the present invention is to provide an optical modulator which is capable of modulating the light amount of incident light having a large beam diameter and which has a high response speed, an output efficiency control device which is capable of obtaining a uniform diffraction effect, and a display apparatus, an infrared sensor and a non-contact thermometer which use the optical modulator and the output efficiency control device.